1. Field of the Invention:
This invention is directed to a method of producing 2,6,6-trimethyl-2-cyclohexene-1,4-dione.
2. Discussion of the Background:
2,6,6-Trimethyl-2-cyclohexene-1,4-dione (ketoisophorone, KIP) is a valuable starting substance for syntheses in terpene chemistry. It can also be employed as a convenient and inexpensive basis for producing vitamin compounds of the A series and vitamin E, and also for producing flavors and fragrances which are identical to natural products (see, for example, Isler, O. 1971, "Carotenoids", pub. Birkhaeuser Verlag, Basel and Stuttgart).
KIP is advantageously synthesized by the catalytic oxidation with oxygen of 2,6,6-trimethyl-2-cyclohexene1-one (alpha-isophorone, alpha-IP) or, preferably, of 3,5,5-trimethyl-3-cyclohexene-1-one (beta-isophorone, beta-IP) (see, Ger. OS 25 26 851, Neth. OS 74 15 848, and Ger. OS 24 57 157 and OS 25 15 304). A problem when using alpha-isophorone is the large number of byproducts which make refinement of the product mixture difficult. For a long time there was no known satisfactory means of producing beta-isophorone. Furthermore, the method is excessively costly if good results are desired in terms of the amount and quality of the KIP product. ##STR1##
Recently the problem of producing beta-isophorone was solved by a new method of isomerizing alphaisophorone (Ger. OS 37 35 211). Therefore the starting product is now available in sufficient quantity and at a satisfactory price.
Beta-isophorone is oxidized to KIP according to Ger. OS 25 15 304 by oxygen or an oxygen-containing gas, with the aid of a lead, vanadium, chromium, manganese, iron, or cobalt salt, optionally in the presence of an organic nitrogen base, e.g. pyridine.
The usual transition metals are less suitable for the reaction, however, because they are particularly good catalysts for the equilibrium reaction between beta-isophorone and alpha-isophorone, and thus their use leads to appreciable back-isomerization (Ger. OS 37 35 211).
In the case of Ger. OS 25 15 304, a large excess of nitrogen base is employed (molar ratio of betaisophorone : pyridine =130 :330), with the aim of increasing yield and suppressing by-products. It is also recommended that a second solvent be added. The eventual removal of the large amount of nitrogen base (by distillation) is attended by high costs.
It is known according to Ger. OS 24 57 157 to use Cu(II) acetylacetonate as a catalyst in the air oxidation of beta-isophorone to KIP; however, the yield is only about 50%.
A need continues to exist for a method of producing KIP from beta-isophorone, with the use of suitable catalysts, which method provides good yields, does not require an additional solvent, and, if possible, enables operation with a reduced amount of nitrogen base.